Computer networks, such as wide area networks (WANs) using the Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite, can provide unicast, multicast, and broadcast services to allow communication between network participants such as a server node and one or more client nodes. IP supports the sending of data to broadcast addresses, which are delivered to all hosts on the network or subnetwork at the expense of creating traffic over the total network. IP also supports multicast technology to communicate over a computer network. The term “broadcast” refers to a server node sending information to all of the client nodes connected to the network. The term “multicast” refers to a server node sending information to a subset of all of the client nodes connected to the network.
Some information providers provide information electronically by broadcasting or multicasting the information from a server node at a central location to one or more client nodes at remote customer locations via a computer network to which the server and the clients are coupled. Computer networks and systems use several different protocols to accomplish reliable data distribution.
TCP/IP, one common protocol suite, presently is used with the Internet. The TCP part provides transport protocol functions to ensure that the total amount of bytes sent is received correctly at the other end. The IP part provides a routing mechanism. In addition, because TCP/IP is a routable protocol, the messages transmitted in accordance with TCP/IP contain the address of a destination network as well as a destination station. Thus, TCP/IP messages can be sent to multiple networks. Conventional TCP/IP reliable file transfer protocols support point-to-point (i.e., unicast) file distribution only. That is, packets are sent to each address in turn.
A protocol within the TCP/IP protocol suite, called User Datagram Protocol (UDP), may be used when reliable delivery of a message is not required (such as video and real-time audio traffic) and thus may offer bandwidth savings. However, because UDP provides no error checking or packet sequence checking, if UDP is used to send messages, then the application using UDP must provide the error checking and packet sequence checking functions.
Most packets today are sent as unicasts (one to one) or broadcasts (one to all). Unicasts have a destination IP address that points to a single recipient. Broadcasts generally have a destination address for all hosts on a specific subnet (a component of a larger network). Multicast is a one to many type of data transmission that permits a sender to transmit a message to multiple recipients at the same time. Multicast is similar to broadcasting, except that multicast generally implies sending information to a list of specific users (e.g., a subset of all possible recipients), whereas broadcast generally implies sending a message to all possible recipients.
One relevant difference between a multicast packet and a unicast (or broadcast) packet is that the destination IP address in a multicast packet refers to an address corresponding to a group of hosts, rather than a specific network or specific host. Thus, multicast traffic on a network can be distinguished from unicast and broadcast traffic by examining the destination IP address, which in a multicast packet identifies the specific multicast group for which an IP packet was sent. For example, if a sender wants to send data to a group of receivers, the sender need only send the data to the particular multicast group address associated with that group. Because receivers in a group can listen at that address for messages, these receivers can receive the packets and process the contents.
Multicast networking and the applications providing one to many services using multicast are becoming important because multicast networks enable applications to scale; that is, multicast enables the applications to service many users without overloading network and sender resources. Widespread usage of applications servicing many users generally is not possible without the scaling provided by efficient network services such as those provided by multicast.
One significant barrier to deployment of multicast, however, has been the reluctance of network operators to actually implement multicast in their network. This often is because it is risky to change networks in any way, for fear of any problems or downtime with the network. Networks already are critical to many businesses and are becoming even more important with time. Thus, network downtime can cost businesses millions of lost dollars and may even cost those responsible their job.
One prior art solution to this problem is known as tunneling. With tunneling, multicast packets are entirely encapsulated inside unicast packets (such as inside TCP) for traversal over parts of the network that do not support multicast. With tunneling, a multicast packet is sent point to point to the end of the tunnel, where the multicast packet is de-encapsulated. FIG. 1 is a simplified example of tunneling. In FIG. 1, a content source 10 (which is multicast enabled) wants to send a multicast message to a receiver (not shown) located on the multicast network 12, but to get there the message must be sent over a non-multicast network 14. To accomplish this, a first router 13 encapsulates the entire multicast packet from source 10 inside a unicast packet. That is, the entire multicast packet, including the content and the source and destination addresses, is encapsulated inside a packet having a different format, such as unicast. The unicast packet is transmitted over the non-multicast network 14, then reaches a second router 13″. This second router 13″ then “strips away” the unicast information, leaving a multicast packet.
Tunneling has a couple of issues that make it non-advantageous to implement on existing networks. The first issue is that tunneling generally is implemented in routers, which requires the network operator to have to reconfigure the routers. Reconfiguring routers risks destabilization of the network. Moreover, if a network operator must reconfigure a router (which the operator generally is loath to do), the operator may simply choose to implement multicast.
Another limitation of tunneling is that it operates in a point to point (i.e., unicast) manner. Thus, in the example of FIG. 1, if the content source 10 wants to send multicast messages to fifty different receivers on the multicast network 12, the source 10 will require fifty different tunnels to be established, each tunnel requiring use of a router at each end. Because tunneling generally requires a fixed tunnel through the non-multicast part of the network, this limits the flexibility of such an approach. For example, there may be cascades of non-multicast parts of the network where the end device may want to split the traffic into multiple unicasts. The ultimate subnets may be multicast enabled, but the non-multicast portion is more complex than a simple point to point link.
Still another issue is one of efficiency. With tunneling, the total multicast packet is encapsulated inside of a unicast packet, resulting in two IP headers of overhead instead of just one, thereby increasing the bandwidth required. Thus, tunneling is very difficult, and sometimes not possible, to implement on certain networks.